The current disclosure is directed to methods and apparatus for the compaction of roadway materials, and more particularly, to methods and apparatus for calibrating a compaction analyzer.
Asphalt is often used as pavement. In the asphalt paving process, various grades of aggregate are used. The aggregate is mixed with asphalt cement (tar), and a paver lays down the asphalt mix and levels the asphalt mix with a series of augers and scrapers. The material as laid is not dense enough due to air voids in the asphalt mix. Therefore, a roller makes a number of passes over the layer of asphalt material, referred to herein as the asphalt mat, driving back and forth, or otherwise creating sufficient compaction to form asphalt of the strength needed for the road surface.
One of the key process parameters that is monitored during the compaction process is the compacted density of the asphalt mat. While there are many specifications and procedures to ensure that the desired density is achieved, most of these specifications require only 3-5 density readings per lane mile. Typically, the density readings will be from extracted roadway cores. The process of measuring density of the asphalt mat during the compaction process is cumbersome, time-consuming, and is not indicative of the overall compaction achieved unless measurements are taken at a large number of points distributed in a grid fashion, which is difficult to achieve in the field due to cost considerations alone. Failure to meet the target density is unacceptable and remedial measures may result in significant cost overruns. Thus, there is a need to develop an intelligent monitoring system that will predict the compacted mat density in real time, over the entire pavement surface being constructed. Because the density cannot be measured directly, researchers have attempted different methods for indirect measurements.
A method that has found some success involves the study of the dynamical characteristics of the vibratory compactors typically used in the field. The compactor and the asphalt mat can be viewed as a mechanically coupled system. An analytical model representing such a system can be used to predict the amount of compaction energy transferred to the mat as a function of frequency (coupled system). The amount of energy transferred can be viewed as a measure of the effectiveness of compaction. The machine parameters, like frequency and speed, can then be altered to maximize the energy transferred, thereby increasing the compaction. However, this method does not yield the compacted density directly; also, relating the energy dissipation to the compacted density is problematic. Therefore, this approach is not suitable to determine the level of compaction of an asphalt roadway.
A number of researchers also tried to study the performance of the compactor during soil and asphalt compaction by observing the vibratory response of the compactor. The vibration energy imparted to the ground (sub-grade soil) during compaction also results in a vibratory response of the compactor. The amplitude and frequency of these vibrations are a function of the compactor parameters and the sub-grade. Thus, the observed vibrations of the compactor can be used to predict the properties of the material being compacted. U.S. Pat. No. 5,727,900 issued to Sandstrom discloses using the frequency and amplitude of vibration of the roller as it passes over the ground to compute the shear modulus and a “plastic” parameter of sub-grade soil. These values are then used to adjust the velocity of the compactor and its frequency and amplitude. Thus, this method attempts to control the frequency of the vibratory motors and the forward speed of the compactor for optimal compaction rather than estimate the density of the compacted soil.
Other methods involve estimating the degree of compaction by comparing the amplitude of the fundamental frequency of vibration of the compactor with the amplitudes of its harmonics. The compactor is instrumented with accelerometers to measure the vibrations of the compactor during operation. By relating the ratio of the second harmonic of the vibratory signal to the amplitude of the third harmonic, the compacted density is estimated with, in some cases, 80% accuracy. These results are encouraging and validate the correlation between the observed vibrations and the property of the material being compacted. However, the accuracy of these techniques needs improvement, as the properties of the asphalt pavement are significantly different at 96.5% and 98% target densities. Further, these methods are susceptible to variations in the data gathered.
Attempts have been made to account for some of the variations seen in the vibratory response of compactors by considering the properties of the mix and the site characteristics, in addition to the vibratory response of the compactor, to estimate density. In one approach a microwave signal is transmitted through the asphalt layer, and the density is estimated based on the transmission characteristics of the wave. While the above techniques have been successful in demonstrating the feasibility of the respective approaches, they need to be further refined before they can be used to predict the density in the field with the required degree of precision.
U.S. patent application Ser. No. 11/271,575 (the '575 application), assigned to the assignee of the present disclosure also provides a method and apparatus for density prediction. In that application, a compactor is utilized to compact a test section, and a vibratory energy is applied to the test section as the compactor moves. Responsive vibratory signals of the compactor are gathered, and the density of the test section is measured with means known in the art, for example, nuclear density gauges, or by cutting cores from the test section and measuring the density of the cores. The vibratory response signals of the compactor are correlated with the measured densities, so that a compaction analyzer can be programmed to generate a signal representative of the measured density when the corresponding vibratory response signal occurs.
The compactor is then utilized to compact an actual roadway section built using roadway material with the same characteristics, and the compaction analyzer will generate density signals based upon the responsive vibratory signals of the compactor. The analyzer will compare the vibratory signals of the compactor to those generated on the test section, and will generate density signals based upon the comparison. In other words, when the analyzer recognizes a vibration signal as the same or similar to that generated on the test section, it will generate a density reading based upon the measurements taken on the test section. While the method and apparatus of the '575 application work well, the construction of an asphalt test mat separate from the roadway being constructed is required, which can be time-consuming and costly.